http://geopo.at/intl/en/
GeoPo is developed by Creco Inc in Japan in 2009. It is designed for Twitter usage. Coding method is not so unique, but GeoPo include http://geopo.at/ as a code. This makes GeoPo code linkable. Creco declare Free License for GeoPo code.

It assumes that whole world is square area in Mercator projection.
Divide it into 8x8=6 hierarchically until required precision.
Use [0-9,A-Z,-_] to express which part of 64 areas. Even one character can means location (or area).

©creco
Sample Code:
http://geopo.at/Z4RHXX